A system disclosed in patent document 1, for example, includes a particulate matter sensor (PM sensor) for detecting particulate matter (hereinafter referred to also as “PM”) in an exhaust passage of an internal combustion engine. The PM sensor includes an insulating substrate and a pair of electrodes disposed on the insulating substrate with a space between the electrodes. When PM in an exhaust gas deposits between the electrodes of the PM sensor, conductivity across the electrodes changes according to an amount of PM deposited. This varies resistance across the electrodes. Detection of a value of the resistance across the electrodes of the PM sensor therefore allows an amount of PM contained in the exhaust gas associated with the amount of PM deposited between the electrodes to be detected.
In the technique disclosed in patent document 1, the PM sensor is disposed downstream of a particulate matter trapping filter (diesel particulate filter, hereinafter referred to also as a “DPF”). In patent document 1, the amount of PM discharged downstream of the DPF is detected based on the value of the resistance across the electrodes of the PM sensor, so that, for example, a determination is made as to whether the DPF is faulty.